This invention relates to an improved lubricating composition, and more particularly, this invention relates to a lubricating composition containing an additive combination having improved antioxidation properties.
Hydrocarbon oils are partially oxidized when contacted with oxygen at elevated temperatures for long periods. The internal combustion engine is a model oxidator, since it contacts a hydrocarbon motor oil with air under agitation at high temperatures. Also, many of the metals (iron, copper, lead, nickel, etc.) used in the manufacture of the engine and in contact with both the oil and air, are effective oxidation catalysts which increase the rate of oxidation. The oxidation in motor oils is particularly acute in the modern internal combustion engine which is designed to operate under heavy work loads and at elevated temperatures.
The oxidation process produces acidic bodies within the motor oil which are corrosive to typical copper, lead, and cadmium engine bearings. It has also been discovered that the oxidation products contribute to piston ring sticking, the formation of sludges within the motor oil and an overall breakdown of viscosity characteristics of the lubricant.
Several effective oxidation inhibitors have been developed and are used in almost all of the conventional motor oils today. Typical of these inhibitors are zinc dithiophosphates and the oil-soluble phenolic and aromatic amine antioxidants. These inhibitors while exhibiting good antioxidant properties, are burdened by economic and oil contamination problems. A need, therefore, exists for an improved antioxidant that is stable at elevated temperatures, that can be employed in reduced concentrations, and that is economical and easy to produce.
It is an object of this invention to provide additive compositions for crankcase lubricating oils which impart improved antioxidant properties. It is a further object of this invention to provide a synergistic additive composition having antioxidant properties in lubricating oil compositions.